xfs: clean up inconsistent variable naming in xfs_swap_extent

The swap extent ioctl passes in a target inode and a temporary inode
which are clearly named in the ioctl structure. The code then
assigns temp to target and vice versa, making it extremely difficult
to work out which inode is which later in the code.  Make this
consistent throughout the code.

Also make xfs_swap_extent static as there are no external users of
the function.

Signed-off-by: Dave Chinner <david@fromorbit.com>
Signed-off-by: Alex Elder <aelder@sgi.com>